1. Field of the Invention
The present invention relates to polyaddition compounds which contain urea groups and uretdione groups and which have free or partially or totally blocked isocyanate groups, to a process for their preparation and to the powdered polyurethane coating materials prepared from them, and to coatings having a matte surface.
2. Description of the Background
There has long been an increasing interest in powder coating materials which produce a matte surface. The reason for this is first and foremost practical. Glossy surfaces require a far higher degree of cleaning than do matte surfaces. In addition, it may be desirable for safety reasons to avoid strongly reflecting surfaces.
The most simple principle of obtaining a matte surface is to admix smaller or larger amounts of a filler such as chalk, finely divided silica or barium sulfate, into the powder coating material in accordance with the extent of the desired matt effect. However, such additions bring about a deterioration in the film properties of the coating, such as adhesion, flexibility, impact strength and chemical resistance.
The addition of substances incompatible with the coating material, such as waxes or cellulose derivatives, for example, does give rise, it is true, to matting, but slight changes in the course of extrusion lead to fluctuations in the surface gloss. The reproducibility of the matte effect is not ensured.
Urea- and uretdione-functional polyisocyanates have become known only in relatively recent times. DE-A 195 46 750 discloses reaction products of isophorone diisocyanate-uretdione and disecondary diamines as curing agents for producing polyurethane (PU) coatings with glossy surfaces.
DE-A 196 30 844 is the first patent application to describe matte PU powder coating materials which comprise urea-functional isophorone diisocyanate-uretdiones as a curing component. They are prepared by reacting uretdiones with water. Intermediates which form during this reaction, with the elimination of carbon dioxide, are primary amines, which react with remaining isocyanate groups to form ureas. The nitrogen atoms of the urea groups are each monosubstituted. Reactions of polyisocyanates with water are difficult to reproduce because of the formation of by-products.
DE A 196 37 375 describes PU powder coating materials which are formulated for a matting effect by means of urea-functional isophorone diisocyanate-uretdiones as curing agents. These curing agents are formed by the reaction of isophorone diisocyanate-uretdione with disecondary diamines. The disecondary diarnines are reaction products of diprimary diamines and maleic or fumaric esters.
DE A 196 37 377 discloses and claims uretdione- and urea-functional polyaddition products as a curing component for matting PU powder coating materials. These curing agents are prepared by the reaction of isophorone diisocyanate-uretdione with diamines which contain one primary and secondary amino group. Preparation of the pure diamines is in some cases complex and very costly.